I125-ECGF recently redesignated heparin binding growth factor-I (HBGF-I) has a very short half-life. These stuies have shown that heparin markedly inhibits proteolytic digestion of HBGF by trypsin and other proteases. This property was lost after thermal denaturation of HBGF, suggesting a heparin - HBGF structural interaction rather than a heparin - trypsin interaction is responsible for the trypsin resistance of HBGF. These data suggest that heparin ameliorates HBGF denaturation and provides conformational stability to the polypeptide growth factor. The stabilizing effect of heparin was dependent upon the concentration of heparin as well as temperature and duration. Autoradiography of 125I-HBGF incubated with human umbilical vein endothelial cells demonstrated near complete inhibition of proteolytic digestion of HBGF when the incubation was performed in the presence of heparin. Inhibitor experiments suggest this serum derived protease is of the metallo-protease class. These data support the concept that the mechanism of the heparin- induced human endothelial cell phenotype involves the protection of HBGF by heparin against inactivation by endothelial cell- derived proteolytic enzymes. Subsequent studies have extended these observations to heparin sulfate, the predominant form of the glycosaminoglycan in the extracellular matrix of the vessel wall, and further suggests a biologic role for this interaction. Low molecular weight heparin fragments with markedly decreased anticoagulant effects also protect HBGF-I , which will be useful for the use of HBGF-I as an infusable agent. Heparin fragments also preserved the bioactivity of HBGF-I maintained at 37 degree C.